1. Field of the Invention
This invention relates to a surface acoustic wave device (SAW device, hereafter), particularly to an improvement of an electrode connecting structure from surface acoustic wave elements (SAW elements, hereafter). This invention also relates to a manufacturing method of a SAW device.
2. Description of the Related Art
Conventionally, SAW devices such as SAW filters are utilized in radio devices of VHF or UHF frequency ranges. For example, SAW filters of 800 to 900 MHz are utilized in high frequency circuits of mobile telephone, mobile radio, MCA and the like.
FIG. 8 shows an example configuration of a SAW device in a cross-sectional view.
In this conventional embodiment, a SAW element 10 is contained in a package consisting of a cap 12 and a base 14.
The SAW element 10, having an input electrode, an output electrode and an earth electrode, and being formed by a material with surface acousticity, is fixed to the metal base 14 by adhesive 16. The base 14 is sealed with the cap 12.
This conventional embodiment belongs to a lead type device. Edges of a fixed number of metal lead pins run through the base 14 and the lead pins 18, 18 are connected to an electrode of the SAW element 10 through gold or copper wires 20, 20 inside the package. Here, the base 14 and the lead pins 18, 18 are electrically insulated, and the base 14 is not electrically connected with the lead pins 18, 18 or the electrodes of the SAW element 10. The other edges of lead pins 18, 18 is connected to a board having an external circuit (not shown in the figure) using through-hole method or the like.
The wires 20, 20 and the electrodes of the SAW element 10 are connected using wire bonding method, which requires pads (parts with designated area and shape) in the electrode of the SAW element 10, as well as a part that is related to transmission and reception of surface acoustic waves.
Recently, other than conventional lead type SAW devices, SAW devices of surface mount type are manufactured and sold in response to the demand of surface mounting of parts.
FIG. 9 shows a conventional embodiment of a surface mount type SAW device.
In the SAW device of this conventional embodiment, a SAW element 110 is stored in a package consisting of a ceramics base 122 and a cover 124 for surface mounting. In this case, electrodes on the upper surface of the SAW element 110 are connected to pads on the surface of the base 122 through a wire 120. In the figure, 116 indicates adhesive.
A SAW device may have more than one input electrode and output electrode. FIG. 10 shows an embodiment of a multiple electrode SAW filter. Multiple electrode SAW filters are widely used for 800-900 MHz range. In the figure, the placement of electrodes on the SAW element is described in detail.
An input electrode 226 and an output electrode 228, both having more than one branch, are adherently attached to the surface of a SAW element 210, which is fixed on a metal base 214 by adhesive (not shown in the figure). The branches are alternately placed on the surface of the SAW element 210.
Earth electrodes 230 and 232, which are formed on the surface of the SAW element 210, are placed at portions near of the branches of the input electrodes 226 and output electrodes 228. Each pair of the input electrode 226 and the earth electrode 230, and the output electrode 228 and the earth electrode 232 constitutes an interdigital electrode. Detail of the interdigital arrangement is not shown in the figure.
The aggregately connected portion of the electrodes 226, 228, 230 and 232, are connected to the edge surface of a lead pin 218 by bonding of a wire 220. The earth electrode 230 and 232 are connected to the base 214 by bonding of a wire 221. The lead pin 218 and the base 214 are electrically insulated by an insulation layer 234.
As a packaging method of electric parts, including SAW devices, face down bonding with a solder bump shown in FIG. 11 is well known. In this method, electronic parts such as IC's are fixed on boards and covered by resin at the same time. For example, the solder bump (a metal bubble) 340 is attached to the designated position of a drawing electrode (not shown in the figure) which is formed on the surface of an IC substrate 342, and the IC substrate 342 is positioned so that the position of the solder bump 340 agrees with the position of a drawing electrode (not shown in the figure). Then, the drawing electrode of the IC substrate is connected to and fixed with the conductor 338 of the printed circuit substrate 336 through the solder bump 340. This can be done, for example, by pressing the IC substrate downward and adding heat which causes melting of the solder. Afterward, the covering is formed by molding with epoxy resin 344. With this method, the surface mount can be performed promptly.
Conventionally, outside dimension of SAW devices tend to become large. In the devices having a configuration of the first or the second conventional embodiment, some play must be given to the wire for easy implementation of the wire bonding, which limits the size of the package, however small the size of the SAW element. Pads used in wire bonding also need to be large enough.
For example, in FIG. 9, suppose that each part has the following dimension: EQU L.sub.P =L.sub.W =L.sub.C =0.5 mm EQU H.sub.W =1.0 mm EQU H.sub.C =0.5 mm
then, the outside dimension of the package L, H with respect to LT and HT are: ##EQU1## where L.sub.T and H.sub.T are the dimensions of the SAW element. Therefore, if a SAW element has a dimension of 1 mm (length).times.2 mm (width).times.0.5 mm (thickness), which is a typical dimension of widely available SAW filters of 800-900 MHz range, the dimension of the package becomes 4 mm (length).times.5 mm (width).times.2 mm (thickness).
Wire bonding also increases the cost. This is because the positioning of the bonding requires high accuracy that demands an expensive manufacturing device. Moreover, in multiple electrode type SAW filters, where the number of bonding is larger, the cost becomes much higher because expensive material such as gold is used as the bonding wire.